Is my conclusion right if a say: Do not buy cheap vari-ND filter, better to buy fix ones 'coz Fix-ND filter should only reduce the amount of light passing through it, and no other effect?
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HolianApr 10 '12 at 19:33

Colour casts are almost impossible to avoid completely; materials simply do not attenuate light evenly at all wavelengths (well, unless they're completely opaque). Even the best-quality ND filters may exhibit some slight colour shift (though often not enough to see with the naked eye)—even "clear" glass usually has some colour bias. The point is that you can correct for the shift easily if it's consistent across the frame.
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user2719Apr 10 '12 at 22:25

3 Answers
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Regardless of price Vari-ND filters always show color-casts which vary in intensity across the frame. The stronger you dial it in, the more color-cast there is. A cheap one will have move oddities but even an expensive vari-ND filter still shows color casts. The color-casts are also such that they are extremely difficult to remove by software.

quality control - cheaper ND filters (even the fixed ones) may be unevenly coated - I've seen reports that you can even see this if you hold the filter up to the light. I don't know that it's any particular brand, but rather some batches will be better than others. If you can buy from a shop where you can inspect (or return a faulty one) that would be best.

stacking filters - if you stack ND filters, and especially if you combine an ND filter with a polarising filter, you can get color casts (purple or orange are often mentioned). I've used very inexpensive ND filters and had no problems until I add a polarising filter into the mix, then got strong, but correctible color casts

polarising filters and Vari-ND filters, used with a wide angle lens, skies will be uneven (lighter or darker depending on the angle of the sun). This isn't really a color cast, but an uneven luminosity across a wide expanse of sky. Not a defect of the filter, just unavoidable physics of light.

if you use Auto white balance, as you add more density, I have found the camera may change the color temperature setting. If you set WB to a preset like Sunny, the issue largely goes away. So the reduction in light hitting the sensor may affect the white balance chosen which may look like a color cast.

Vari-ND filters, when used with wide angle lenses and near maximum density, can exhibit a cross effect (see the blog post for sample images). This again is not really a defect in the filter per se, but an unavoidable effect caused by using a pair of polarisers.

Now the different brands aren't going to advertise color casts. You can look on Amazon review and dpreview forums and hope to get an idea from comments. Again, if you can buy from a shop where you can inspect the filter, try it out, or return it if it doesn't perform, might be worth the extra cost vs buying one online.

Even with potential color casts (which you could correct by using a gray card), the versatility and convenience of the vari ND filters is worth it in my opinion. You can leave the filter on the lens, rotate it to minimum density, focus and compose, then darken it to suit and take your exposures, and repeat. Compared to fixed ND filters where you have to remove them in order to focus, then replace them.

But if you want absolute best quality, you're probably better off with fixed ND filters.